This invention relates to a process for pretreating hydrocarbon feed stocks as described in my pending U.S. patent application, Ser. No. 90,247, filed Nov. 1, 1979, entitled "Upgrading Petroleum and Residual Fractions Thereof." This invention particularly relates to an improvement upon the method for controlling a process for pretreating hydrocarbon feed stocks as described in my pending U.S. patent application, Ser. No. 144,477, filed Apr. 28, 1980, entitled "Selective Vaporization Process And Dynamic Control Thereof."
In my pending application Ser. No. 90,247, a process is described for increasing the portion of heavy petroleum crudes which can be utilized as the hydrocarbon feed stocks for fluid catalytic cracking ("FCC") processes to produce premium petroleum products, particularly motor gasoline of high octane number or high quality heavy fuel. The heavy ends of many crudes are high in Conradson Carbon residues (sometimes reported as Ramsbottom Carbon residues) and metal values, such as nickel and vanadium, as well as salts, such as sodium salts, which are undesirable in FCC feed stocks and in products such as heavy fuel. The process of my application Ser. No. 90,247 provides an economically attractive method for selectively removing and utilizing these undesirable components from whole crudes, as well as from the bottom fractions or residues of atmospheric and vacuum distillations of whole crudes, commonly called atmospheric and vacuum residua or "resids". In this regard, terms such as "residual stocks" and "resids" are used in a somewhat broader sense than is usual to include any petroleum fraction remaining after fractional distillation of petroleum to remove some of its more volatile components. In that sense, "topped crude", remaining after distilling off gasoline and lighter fractions, is a resid. The undesirable high Conradson Carbon (low hydrogen content) compounds, such as polynuclear aromatic compounds, and metal-containing compounds, as well as salts, present in crudes (e.g., whole crudes or resids) tend to be concentrated in the resids because most of them have low volatility.
When first introduced to the petroleum industry in the 1930's, the FCC process constituted a major advance over previous processes for increasing the yield of motor gasoline from petroleum to meet ever increasing demands. The FCC process was adapted to produce abundant yields of high octane naphtha from petroleum fractions boiling above the gasoline range, upwards of about 400.degree. F. Greatly improved FCC processes have since been developed by intensive research efforts, and plant capacity has expanded rapidly up to the present, so that the catalytic cracker is today the dominant unit or "workhorse" of a petroleum refinery.
As installed capacity of FCC processes has increased, there has been increasing pressure to charge, as feed stocks to FCC units, greater proportions of crudes. However, two major factors have opposed that pressure, namely, the Conradson Carbon residues and metal values in the crudes. As the Conradson Carbon residues and metal values have increased in crudes, charged to FCC processes, cpacity and efficiency of catalytic crackers have been adversely affected. Also, the quality of heavy fuels, such as Bunker Oil and heavy gas oil, produced by FCC processes, has also been adversely affected as it has become necessary to make these fuels from crudes of high Conradson Carbon residues and high metal values.
The effect of high Conradson Carbon residues in hydrocarbon feed stocks for FCC processes has been to increase the portion of the feed stocks converted to "coke" deposits on the FCC catalysts. As coke has built up on the FCC catalysts, the active surfaces of the catalysts have been masked and rendered inactive for the desired catalytic cracking. It has been conventional to burn off the inactivating coke with air to "regenerate" the active surfaces, after which the catalysts have been returned in cyclic fashion to the reaction stage for contact with, and cracking of, additional feed stocks. The heat generated in the regeneration stage has been recovered and used, at least in part, to supply the heat of vaporization of the feed stocks and the endothermic heat of the cracking reaction. The regeneration stage has operated under a maximum temperature limitation to avoid heat damage to the catalysts. Since the rate of coke burning is a function of temperature, it has followed that any regeneration stage has had a limit of coke which could be burned in unit time. As the Conradson Carbon residues in feed stocks have increased, coke burning capacity has become a bottle-neck which has forced a reduction in the rate of charging the feed stocks to FCC units. In addition, part of the feed stocks have inevitably had to be diverted to undesirable reaction products.
Metal values, such as nickel and vanadium, in hydrocarbon feed stocks for FCC processes have tended to catalyze the production of coke and hydrogen in FCC units. Such metals also have tended to be deposited on FCC catalysts, as the molecules in which they occur in the feed stocks are cracked, and to build up on the catalysts. This has further increased coke production with its accompanying problems. Excessive hydrogen production also has caused a bottle-neck problem in processing lighter ends of cracked products through fractionation equipment to separate valuable components, primarily propane, butane and the olefins of like carbon number. Hydrogen, being incondensible in the "gas plant", has occupied space as a gas in the compression and fractionation train and has tended to overload the system when excessive amounts are produced by high metal content catalysts. This has required a reduction in charge rates to maintain FCC units and their auxiliaries operative.
These problems have long been recognized in the art, and many ways have been proposed to remove the high Conradson Carbon and metal-containing components from hydrocarbon feed stocks, such as resids, before they are used in FCC processes. For example, the high Conradson Carbon and metal-containing components of resids have been thermally converted into large quantities of solid fuel, i.e., coke. Such processes have been termed "coking", and two types of coking are now practiced commercially. In delayed coking, the resids have been heated in a furnace and passed to large drums maintained at 780.degree.-840.degree. F. During the long residence time at such temperatures, the resids have been converted to coke, and distillate products have been taken off the top of the drum for recovery of "coker gasoline", "coker gas oil" and gas. The other coking process has employed a fluidized bed of small granules of coke at about 900.degree.-1050.degree. F. The resids have undergone conversion on the surface of the coke particles during a residence time on the order of 2 minutes, depositing additional coke on the surfaces of the coke particles in the fluidized bed. Coke particles have then been transferred to a bed fluidized by air to burn some of the coke at temperatures upwards of 1100.degree. F., thereby heating the residual coke which has then been returned to the coking vessel for conversion of additional resid.
These coking processes have been known to reduce metal values and Conradson Carbon residues in resids but to leave the resids as inferior gas oils for charging to FCC units. The coking processes have induced extensive cracking of components in the resids which would otherwise have been valuable in FCC feed stocks. This has resulted in gasoline of lower octane number (from thermal cracking) than would have been obtained by subjecting the same components to FCC processes. The gas oils produced have been olefinic and contained significant amounts of diolefins which are prone to degradation to coke in furnace tubes and on FCC catalysts. Hence, it has often been desirable to treat the gas oils by expensive hydrogenation techniques before charging them to FCC units or blending with other fractions for fuels.
Feed stocks for FCC processes have also been prepared from resids by "deasphalting" in which an asphalt precipitant such as liquid propane has been mixed with the resids. Metal values and Conradon Carbon residues of the resids have been drastically reduced, but low yields of deasphalted oil have been obtained.
Solvent extractions and various other techniques have been proposed for preparation of FCC feed stocks from resids. Low temperature, liquid phase sorption on catalytically inert silica gel also has been proposed by Shuman and Brace, Oil and Gas Journal, page 113 Apr. 6, 1963). However, such processes have had very high energy costs and have only been useful where the asphalt by-products could be sold.
By the pretreatment process of my pending application Ser. No. 90,247, high Conradson Carbon and metal-containing components, as well as salts, can be economically removed from a hydrocarbon feed stock, containing the highest boiling components of a crude, before charging the feed stock to an FCC unit or a hydroprocessing unit. In this pretreatment process, the feed stock is subjected to a selective vaporization step in which there is a high temperature, short hydrocarbon residence time contact in a confined rising vertical column between the feed stock and a hot solid contact material. The contact material serves as a heat transfer medium and acceptor of unvaporized material from the feed stock. The contact material is essentially inert in the sense that it has low catalytic activity for inducing cracking of the feed stock. Preferably, the contact material has a much lower surface area relative to its weight than conventional FCC catalysts. During the selective vaporization step, most of the feed stock is vaporized by the high temperature contact with the contact material. However, the majority of the high Conradson Carbon and metal-containing components of the feed stock, as well as salts in the feed stock, are not vaporized by the high temperature contact with the contact material but are instead deposited on the surface of the contact material. The contact material, on which the unvaporized portions of the feed stock have been deposited, is then subjected to a combustion step in which the combustible portions of the deposits on the contact material are oxidized to generate heat which is imparted to the contact material. The so-heated contact material is then recycled and contacted with additional feed stock. By this process, the heat required for the selective vaporization step is generated by oxidation of the combustible deposits on the contact material, including the combustible high Conradson Carbon and metal-containing components of the feed stock.
By the method of my pending application Ser. No. 144,477 for controlling my pretreatment process, the selective vaporization step is carried out at about the minimum contact temperature which: (a) will vaporize most of the hydrocarbon feed stock and its diluents but not the majority (e.g., 60-80%) of its high Conradson Carbon components, its metal-containing components or its salts; and (b) will provide sufficient combustible deposits of unvaporized portions of the feed stock on the contact material to allow the combustion step to be carried out at a predetermined temperature. The predetermined temperature of the combustion step is preferably set at or near the maximum allowable temperature of the combustion step. This is usually governed by the metallurgical limits of the burner in which the combustion step is carried out. The rate of recycle from the combustion step to the selective vaporization step of the contact material, heated during the combustion step, is varied to maintain the selective vaporization step at the minimum contact temperature which will provide sufficient combustible deposits on the contact material to allow the combustion step to be carried out at its predetermined temperature.
However, it has been found that, in the selective vaporization step of the pretreatment process of my pending application Ser. Nos. 90,247 and 144,477, metals, such as nickel and vanadium, which are deposited, and build up, on the surface of the contact material in the selective vaporization step, have tended to combine chemically with the contact material during the combustion step of the process. This has made separation of such metals from the contact material difficult and has caused the contact to be degraded during processes, used to remove the metals from the contact material.
There has been a need, therefore, for a way of preventing metals, deposited on the contact material, from chemically combining with the contact material during the combustion step of the process.